SCIENCE RESEARCH LABORATORY PROPOSES TO DESIGN, CONSTRUCT AND TEST AN ULTRA-SENSITIVE LASER INTERFEROMETER TO MEASURE THE PLASMA DENSITY INSIDE A PLASMA EROSION OPENING SWITCH DURING ITS CONDUCTION AND OPENING PHASES. A SIMILAR INTERFEROMETER HAS ALREADY BEEN PROVEN BY SRL FOR ULTRA-SENSITIVE GAS ABSORPTION MEASUREMENTS. THE HIGHER BANDWIDTH REQUIRED FOR PEOS MEASUREMENTS AND THE MORE HOSTILE PULSED POWER ENVIRONMENT DEMAND A HIGHER POWER LASER, WHICH IS COMMERCIALLY AVAILABLE. THE DENSITY MEASUREMENTS PROPOSED HERE, WITH MILLIMETER SPATIAL RESOLUTION AND NANOSECOND TEMPORAL RESOLUTION, WILL BE THE FIRST OF THEIR KIND IN TERAWATT OPENING SWITCHES. BENCH TESTS OF FLASHBOARD PLASMAS WILL BE FOLLOWED BY FIELD TESTS ON A TERAWATT CLASS OPENING SWITCH TEST-BED. MORE ACCURATE SCALING LAWS FOR HIGHER PERFORMANCE OPENING SWITCH DESIGNS WILL RESULT FROM THESE DETAILED TIME-RESOLVED DENSITY MEASUREMENTS. THIS PROJECT COULD HAVE MAJOR IMPACT ON THE DESIGN OF THE PROPOSED DECADE X-RAY SIMULATOR. THE INTERFEROMETER BUILT HERE CAN ALSO BE USED TO CHARACTERIZE PLASMA BLOW-OFF FROM ELECTRODE SURFACES IN OTHER REGIONS OF HIGH POWER ACCELERATORS FOR FREE ELECTRON LASERS, INERTIAL FUSION OR X-RAY SIMULATORS, SUCH AS MAGNETICALLY INSULATED TRANSMISSION LINES, PARTICLE BEAM DIODES AND X-RAY DIODES.